PETN – the explosive that preliminary tests indicate was packed in the shoes of wannabe suicide bomber Richard Reid – is a fairly uncommon and powerful compound often used as a military detonator.

Close to half a pound of PETN was discovered in Reid’s sneakers after he was subdued on Saturday by passengers and crew on the trans-Atlantic flight.

PETN, a white crystalline substance, is most commonly used as a detonator for military shells, mines and bombs.

It is difficult to manufacture and has limited civilian use. PETN can be obtained in the United States for about $20 a pound by people with blasting-cap permits.

PETN is a key ingredient in plastic explosives like Semtex, which was used to blow up Pan Am Flight 103 over Lockerbie, Scotland in 1988.

In 1996, after the Paris-bound TWA Flight 800 exploded over the Atlantic, FBI revealed traces of PETN were detected in the wreckage, leading many to believe that the plane went down as a result of foul play – a theory which the FBI has avoided.

The FBI’s then-assistant director, James Kallstrom, said at the time that “the mere fact that there are chemical traces is just not enough [to prove that explosives brought the plane down.]”

Kallstrom, appointed New York state’s public-security director in the wake of Sept. 11, said it was possible that the PETN could have been brought on the plane by a passenger and was not part of a bomb.